This invention generally relates to protection systems for alternating current (ac) generators. More particularly, this invention relates to systems for protecting ac generators of diesel-engine generator sets against damage from overload.
The protection of ac generators of diesel-engine generator sets against damage from overload is presently accomplished by one of several arrangements. In a first arrangement, a measurement of the ac generator field current is used to shut down the excitation field of the ac generator when the field current increases above a predetermined value. In a second arrangement, a circuit breaker is provided in the output circuit. Both arrangements have proved unsatisfactory for several reasons. Where the field current is measured, the currents being measured are typically small; e.g. two amperes. Depending on the output voltage and the load, normal field current variation can be larger than the value to which the detector is set to shut down excitation. Thus, the diesel-engine generator may be inadvertently shut down during normal operation; an undesired situation.
When a circuit breaker in the output circuit is used, the current in the load is measured by a thermal or magnetic element, and the circuit breaker is tripped (i.e. the circuit is opened) when the load current exceeds a given value for a certain length of time. When used in compressor motor starting, as is the case with transport refrigeration units, the circuit breaker must not trip immediately as the motor starting ("inrush") current must be accommodated. Typically the inrush current is about five to six times the normal load current, and must be accommodated for a period of between one to five seconds. Because of the materials and components used in typical circuit breakers designed for motor starting, a range of circuit breaker trip times is expected for a given overload current. For example, as seen in the prior art FIG. 1, the circuit breaker trip time for a 700% current overload is typically one to four seconds. In diesel-engine generator sets for 460 V service for refrigerated containers, the typical circuit breaker rating is thirty amperes, and a 700% overload represents two hundred ten amperes. When such a circuit breaker is used with a power grid network of a public utility, the grid is able to supply well in excess of two hundred ten amperes. The circuit breaker can therefore be expected to open the circuit within the one to four seconds in the event of a short circuit in such situations. However, with small generators, such as are used in diesel-engine generator sets, the maximum current that can be delivered by the ac generator is about one hundred fifty amperes, or 500% overload. In such situations, as seen in FIG. 1, the trip time of the circuit breaker can be as long as approximately eight seconds at which time the ac generator may already be damaged. Moreover, the circuit breakers of the art will not protect the ac generator in time in the case of current imbalance; i.e. where one of the conductors of a three phase circuit is not drawing current (single phase condition) and the unit is running at only 500% overload or less.
In light of the shortcomings of the field current measurement and the circuit breaker in the output circuit arrangements, it appears that the only reliable manner of protecting the ac generator is to measure the load current directly. Circuit breakers which measure the load current and cause the circuit breaker to trip with an auxiliary device are known in the art. They are not suitable for application to diesel-engine generator set protection, however, because they do not have adequate delay time to accommodate motor starting currents.